Old enough to be an adult with kids.
Luckily for them, I had already dug into enough safety study data to realize that they are never tested on infants for ethical reasons.
Safety is therefore implied from adult test subjects.
So I dug into infant immune system research and very quickly discovered that they're structured and behave very differently from adults.
Then I discovered that the only reason they administer them to infants and toddlers is for administrative convenience.
So we hit pause until the kid's immune systems matured. And in the meantime it became clear that these things are not at all just what they present themselves to be.